Effect of Coulomb Correlation on the Magnetic Properties of Mn Clusters

In spite of decades of research, a fundamental understanding of the unusual magnetic behavior of small Mn clusters remains a challenge. Experiments show that Mn₂ is <i>antiferromagnetic</i> while small clusters containing up to five Mn atoms are ferromagnetic with magnetic moments of 5 μ_B/atom and become ferrimagnetic as they grow further. Theoretical studies based on density functional theory (DFT), however, find Mn₂ to be <i>ferromagnetic</i>, with ferrimagnetic order setting in at different sizes that depend upon the computational methods used. While quantum chemical techniques correctly account for the antiferromagnetic ground state of Mn₂, they are computationally too demanding to treat larger clusters, making it difficult to understand the evolution of magnetism. These studies clearly point to the importance of correlation and the need to find ways to treat it effectively for larger clusters and nanostructures. Here, we show that the DFT+<i>U</i> method can be used to account for strong correlation. We determine the on-site Coulomb correlation, Hubbard <i>U</i> self-consistently by using the linear response theory and study its effect on the magnetic coupling of Mn clusters containing up to five atoms. With a calculated <i>U</i> value of 4.8 eV, we show that the ground state of Mn₂ is <i>antiferromagnetic</i> with a Mn–Mn distance of 3.34 Å, which agrees well with the electron spin resonance experiment. Equally important, we show that on-site Coulomb correlation also plays an important role in the evolution of magnetic coupling in larger clusters, as the results differ significantly from standard DFT calculations. We conclude that for a proper understanding of magnetism of Mn nanostructures (clusters, chains, and layers) one must take into account the effect of strong correlation

Efficient Carrier Separation and Band Structure Tuning of Two-Dimensional C₂N/GaTe van der Waals Heterostructure

Efficient carrier separation and suitable band structure are critical for developing better nanoscale optoelectronic devices. However, so far, researchers have not developed a single material system that can satisfy these requirements. Here we design a novel C₂N/GaTe van der Waals heterostructure based on the density functional theory. Our results suggest that this heterostructure is an indirect band gap semiconductor (1.39 eV) with intrinsic type-II band alignment, facilitating the separation of photogenerated carriers. Meanwhile, this heterostructure exhibits improved visible optical absorption compared with that of the isolate C₂N and GaTe monolayers. More fascinatingly, we find that an intriguing indirect-to-direct band gap semiconductor transition can be induced at the compressive strain of 3%. Simultaneously, the band gaps and carrier effective masses can also be significantly reduced by the biaxial strain. Furthermore, the band edge positions of C₂N/GaTe heterostructure can be effectively tuned to straddle the redox potentials of water splitting by isoelectronic anion S and Se substitution at the Te site, and the enhanced optical absorptions are also observed in the doped heterostructures, indicating that S (Se)-doped C₂N/GaTe heterostructures are potential photocatalysts for water splitting. In addition, effective spatial separation of photogenerated carriers is expected to occur for all of the above cases. These findings suggest that the C₂N/GaTe heterostructure is a promising candidate for application in future nanoelectronics and optoelectronics devices and also provides some valuable information for future experimental research

New Ferroelectric Phase in Atomic-Thick Phosphorene Nanoribbons: Existence of in-Plane Electric Polarization

Ferroelectrics have many significant applications in electric devices, such as capacitor or random-access memory, tuning the efficiency of solar cell. Although atomic-thick ferroelectrics are the necessary components for high-density electric devices or nanoscale devices, the development of such materials still faces a big challenge because of the limitation of intrinsic mechanism. Here, we reported that in-plane atomic-thick ferroelectricity can be induced by vertical electric field in phosphorene nanoribbons (PNRs). Through symmetry arguments, we predicted that ferroelectric direction is perpendicular to the direction of external electric field and lies in the plane. Further confirmed by the comprehensive first-principles calculations, we showed that such ferroelectricity is induced by the electron-polarization, which is different from the structural distortion in traditional ferroelectrics and the recent experimental discovery of in-plane atomic-thick ferroelectrics (<i>Science</i> <b>2016</b>, <i>353</i>, 274). Moreover, we found that the value of electronic polarization in bilayer is much larger than that in monolayer. Our results show that electron-polarization ferroelectricity maybe the most promising candidate for atomic-thick ferroelectrics

Quantum Phase Transition in Germanene and Stanene Bilayer: From Normal Metal to Topological Insulator

Two-dimensional (2D) topological insulators (TIs) that exhibit quantum spin Hall effects are a new class of materials with conducting edge and insulating bulk. The conducting edge bands are spin-polarized, free of back scattering, and protected by time-reversal symmetry with potential for high-efficiency applications in spintronics. On the basis of first-principles calculations, we show that under external pressure recently synthesized stanene and germanene buckled bilayers can automatically convert into a new dynamically stable phase with flat honeycomb meshes. In contrast with the active surfaces of buckled bilayer of stanene or germanene, the above new phase is chemically inert. Furthermore, we demonstrate that these flat bilayers are 2D TIs with sizable topologically nontrivial band gaps of ∼0.1 eV, which makes them viable for room-temperature applications. Our results suggest some new design principles for searching stable large-gap 2D TIs

Two-Dimensional Hexagonal Transition-Metal Oxide for Spintronics

Two-dimensional materials have been the hot subject of studies due to their great potential in applications. However, their applications in spintronics have been blocked by the difficulty in producing ordered spin structures in 2D structures. Here we demonstrated that the ultrathin films of recently experimentally realized wurtzite MnO can automatically transform into a stable graphitic structure with ordered spin arrangement via density functional calculation, and the stability of graphitic structure can be enhanced by external strain. Moreover, the antiferromagnetic ordering of graphitic MnO single layer can be switched into half-metallic ferromagnetism by small hole-doping, and the estimated Curie temperature is higher than 300 K. Thus, our results highlight a promising way toward 2D magnetic materials

Half-Metallicity in Organic Single Porous Sheets

The unprecedented applications of two-dimensional (2D) atomic sheets in spintronics are formidably hindered by the lack of ordered spin structures. Here we present first-principles calculations demonstrating that the recently synthesized dimethylmethylene-bridged triphenylamine (DTPA) porous sheet is a ferromagnetic half-metal and that the size of the band gap in the semiconducting channel is roughly 1 eV, which makes the DTPA sheet an ideal candidate for a spin-selective conductor. In addition, the robust half-metallicity of the 2D DTPA sheet under external strain increases the possibility of applications in nanoelectric devices. In view of the most recent experimental progress on controlled synthesis, organic porous sheets pave a practical way to achieve new spintronics

Hexagonal Boron Nitride with Designed Nanopores as a High-Efficiency Membrane for Separating Gaseous Hydrogen from Methane

Using first-principles calculations and molecular dynamics simulations, we theoretically explored the potential applications of hexagonal boron nitride (h-BN) for H₂/CH₄ separation. The h-BN with appropriate pores possesses excellent H₂/CH₄ selectivity (>10⁵ at room temperature). Furthermore, the adsorption energies (0.1 eV more or less) of both H₂ and CH₄ on the designed monolayer membranes are sufficiently low to prevent the blocking of the nanopores in a realistic separating process. Particularly, we demonstrate a highly promising membrane (h-BN with a triangular pore and a N9H9 rim) with a calculated diffusion barrier of 0.01 eV for H₂ diffusion, and the simulated flux of H₂ across the single layer is as large as 4.0 × 10⁷ GPU at 300 K. Additionally, the estimated permeability of H₂ significantly exceeds the industrially accepted standard for gas separation over a broad temperature range. Therefore, our results suggest that porous boron nitride nanosheets will be applicable as new membranes for gas separation

Geometric and Electronic Structures as well as Thermodynamic Stability of Hexyl-Modified Silicon Nanosheet

The successful synthesis and outstanding properties of graphene have promoted strong interest in studying hypothetical graphene-like silicon sheet (silicene). Very recently, 2D silicon nanosheet (Si-NS) stabilized by hexyl groups was reported in experiment. We here present an atomic-level investigation of the geometric stability and electronic properties of Si-NS by density functional calculations and molecular dynamics simulations. The most stable structure of the hexyl-modified Si-NS corresponds to the one in which the hexyl groups are regularly attached to both sides of the sheet, with the periodicity of the hexyl groups on the sheet being 7.17 Å, in good agreement with the experimental value of 7.1 Å. The electrostatic repulsion effect of the hexyl groups could be an important reason for the favorable structure. The electronic structure of the hexyl-modified Si-NS shows a direct band gap that is not sensitive to the length of the alkyl group but sensitive to the strain effect, which can be used to tune the gap continuously within the whole strain range we considered. Finally, both the first-principles and the force-field-based molecular dynamics simulations show that the most stable structure of the hexyl-modified Si-NS could maintain its geometric configuration up to 1000 K

Prominently Improved Hydrogen Purification and Dispersive Metal Binding for Hydrogen Storage by Substitutional Doping in Porous Graphene

By density functional theory calculations, we demonstrate that the high selectivity for H₂ permeability relative to CH₄, CO, and CO₂ could be fine adjusted by B or N doping in porous graphene (PG), which is very useful for separation of H₂ from the mixed gases. Also, the atomically dispersed Li and Ca bindings to the polyphenylene structure are significantly enhanced by B doping. The average binding energies for fully adsorbed Li and Ca atoms on 2B-PG of 1.62 and 1.75 eV are greatly larger than 0.68 and 1.05 eV for pure PG, respectively. It is beneficial to experimental metal decoration since these values exceed the cohesive energies per atom of bulk Li and Ca. Grand canonical Monte Carlo simulations show that the high H₂ storage capacities with 6.4 wt % for Li-decorated 2B-PG and 6.8 wt % for Ca-decorated 2B-PG can be obtained at 298 K and 100 bar. Thus, PG through successful controlled synthesis and available doping technology will be expected to achieve the coming hydrogen economy

Crystallographic Facet Dependence of the Hydrogen Evolution Reaction on CoPS: Theory and Experiments

Cobalt phosphosulfide (CoPS) has recently emerged as a promising earth-abundant electrocatalyst for the hydrogen evolution reaction (HER). Nonetheless, the influence of crystallographic surface on the HER activity of CoPS and other nonmetallic electrocatalysts remains an important open question in the design of high-performance catalysts. Herein, the HER activities of the (100) and (111) facets of CoPS single crystals were studied using complementary experimental and computational approaches. Natural (111) and polished (100) facets of CoPS single crystals were selectively exposed to reveal that the HER behaviors on these two facets are quite different, with current density–potential curves crossing near 0.35 V vs RHE. Computational analysis can explain this phenomenon in terms of strongly differing H atom adsorption free energies and H–H recombination barriers on the facets, in conjunction with a simple kinetic model. At low potential (0–0.35 V), H adsorption (Volmer step) is rate limiting due to the endergonic adsorption on the (111) facet vs exergonic adsorption on the (100) facet, yielding a faster HER rate for the latter. However, at high potential (>0.35 V), H₂ recombination/desorption becomes limiting and thus the (111) facet, with lower associated barriers, shows better HER activity. Explicit consideration of both steps and their interplay allows for a comprehensive description of the overpotential-dependence of the HER activity. This integrated study yields additional insight into the factors which govern the facet-dependence of catalytic activity on nonmetallic electrocatalysts and can further improve the design of advanced nanostructured HER catalysts